Catalyst and reverse disproportionation process

ABSTRACT

A catalyst containing tungsten, iridium, and an alkali or alkaline earth component, preferably potassium, on a support, preferably silica gel, is disclosed which is useful in reverse disproportionation of stilbene and ethylene to produce styrene.

This is a division of application Ser. No. 403,253, filed July 29, 1982now U.S. Pat. No. 4,440,968.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for reversedisproportionation of ethylene and stilbene to produce styrene. Thecatalyst comprises tungsten, potassium, and iridium on a support,preferably silica gel.

2. Description of the Prior Art

The production of styrene from stilbene and ethylene is disclosed inU.S. Pat. No. 3,965,206, the teachings of which are incorporated byreference. Use of conventional disproportionation catalysts such ascobalt molybdate on alumina, or tungsten oxide or silica, alumina orsilica-alumina, for reverse disproportionation is taught.

U.S. Pat. No. 3,764,635, Fattore, et al, the teachings of which areincorporated by reference, teaches a process for disproportionatingolefins using a catalyst of tungsten and bismuth on a support,preferably silica.

U.S. Pat. No. 3,792,107, Fattore, et al, the teachings of which areincorporated by reference discloses use of a catalyst of tungsten andcopper or tungsten and Group VIII metals, preferably Fe, Co or Ni, onsilica or other support. It is claimed that this catalyst requires noactivation before use in disproportionation.

U.S. Pat. No. 3,728,414, Helden, et al, the teachings of which areincorporated by reference, teaches a conventional olefindisproportionation catalyst with a promoter, a Group IIIa metal on analumina carrier. Conventional olefin disproportionation catalysts aresaid to contain titanium, vanadium, chromium, manganese, zirconium,niobium, molybdenum, technetium, ruthenium, rhodium, palladium, tin,hafnium, tantalum, tungsten, rhenium, osmium, and iridium. Thisreference teaches that additional components, e.g., coactivators,hydrogenating components, components for isomerization of the doublebond, and the like may also be added. Coactivators listed include cobaltoxide, and compounds of iron, nickel, and bismuth.

U.S. Pat. No. 4,192,961 teaches conversion of a mixture of dibenzyl andstilbene with ethylene in the presence of a catalyst of chromium oxide,tungsten oxide, an oxide of an alkali metal and silica or alumisilicate.Styrene yields of 78 to 80 weight percent, based upon conversion ofethylbenzene, dibenzyl and stilbene, are claimed.

U.S. Pat. No. 3,658,930, Kenton, et al, the teachings of which areincorporated by reference, teaches disproportionation of olefins using arhodium oxide promoter on conventional olefin disproportionationcatalyst, e.g., tungsten, molybdenum, rhenium or tellurium on silica.

U.K. Patent Specification No. 1,205,677 teaches disproportionation ofolefins using a conventional catalyst, such as molybdenum trioxide,tungsten trioxide or rhenium heptoxide on alumina, silica, oralumina-silica, and incorporation into this conventional catalyst asecond component to effect double bond isomerization of olefins. GroupVIII noble metals are suggested as being suitable, with preferredisomerization catalysts containing platinum and especially palladium. Analkali or alkaline earth metal ions are added to the catalyst to serveas a base to inhibit the oligomerization of branched chain olefins.

None of these prior art catalysts are believed to possess sufficientactivity and stability to permit their use in a commercial reversedisproportionation process.

Another failing of most prior art catalysts, is that a relatively hightemperature activation procedure is necessary before the catalysts aresuitable for use. These catalysts are extremely active, but have veryshort lives before carbon and coke deposition destroys catalyticactivity. Frequent regeneration and activation of the catalyst arenecessary for a successful commercial process. It is desirable tominimize stress on the catalyst, and on the equipment by eliminatinglarge temperature swings necessary for activation and regeneration ofthe catalyst. It is also desirable if the catalyst has great stability,and is able to operate for relatively long periods.

SUMMARY OF THE INVENTION

The present invention provides a catalyst comprising catalyticallyeffective amounts of iridium, tungsten, and an alkali or alkaline earthcomponent on a carrier material.

In another embodiment, the present invention provides a process for thereverse disproportionation of stilbene and ethylene which comprisescontacting stilbene and ethylene at reverse disproportionationconditions with an activated catalyst containing iridium, tungsten, andan alkali or alkaline earth component or compounds thereof supported ona carrier material.

In a more limited embodiment, the present invention provides a processfor the reverse disproportionation of stilbene and ethylene into styrenecomprising contacting the stilbene and ethylene at temperature of 300 to600 C. with an activated catalyst comprising tungsten, iridium, and analkali or alkaline earth metal component or compound thereof on silicagel carrier, and wherein the atomic ratio of iridium to tungsten is from1:20 to 2:1, to produce styrene, and continuing said contact until saidcatalyst has been at least partially deactivated by coke deposition,removing said deactivated catalyst from contact with reactants andregenerating said catalyst by oxidizing coke from said catalyst with anoxygen containing gas to produce an oxidized catalyst with reduced cokecontent and thereafter activating said catalyst by contacting saidoxidized catalyst with activating gas at 400 to 600 C. for a timesufficient to activate said catalyst, and thereafter returning saidcatalyst to contact with stilbene and ethylene for further reversedisproportionation of stilbene and ethylene into styrene.

DETAILED DESCRIPTION

The Reverse Disproportionation Reaction

The total reaction of this invention may be represented by the followingequation: ##STR1##

Catalyst

The catalyst may contain from 0.1 to 10 wt % W, preferably 1 to 6 wt %,and 0.01 to 2 wt % Ir, preferably 0.1 to 1 wt %. The catalyst also has0.01 to 2 wt %, preferably 0.03 to 0.3 wt % alkali or alkaline earthmetal ion, preferably potassium. Other promoters may be present.

The support is preferably silica gel, but any other support used forconventional disproportionation catalyst may also be used, though thecatalyst performance may change some.

Activation

Activation is necessary to achieve the catalyst's full potential.Activation is a partial reduction of the catalytic components, which areoxides because of the calcination step used in catalyst manufacture, orbecause of the oxidizing atmosphere used to burn off coke on spentcatalyst. Conventional activation procedures, such as used forconventional disproportionation catalysts may be used.

Reaction Conditions

The reverse disproportionation reaction conditions are given in U.S.Pat. No. 3,965,206, the teachings of which are incorporated byreference. In general, temperatures of 300 to 600 C. are adequate.Pressures from subatmospheric to 1000 atm, absolute are suitable, butoperation at 1 to 10 atmospheres gives good results.

Hydrogen or nitrogen or inerts may be present during thedisproportionation reaction. Adding hydrogen may or may not cut down oncatalyst cooking, but may overreduce the catalyst. Nitrogen, andhydrogen and other inert gases, will also cut down resident time ofreactants in the reactors, if desired. I prefer to operate withreactants are the sole feed to the reactor.

The feed to the process of the present invention consists of relativelystilbene and ethylene. Other materials may be present, but polarmaterials act as catalyst poisons.

EXAMPLES Reactor

The experimental apparatus used in all examples consisted of a 0.5-inchOD stainless steel tube, 18-31 cm long. The catalyst was maintained inthe reactor as a fixed bed. Reactants flowed in a vapor phase, downflow, through the catalyst bed. The catalyst was supported on a quartzwool plug resting on an inert support. During the early phases of thestudy, 1/8-inch alundum beads were used, but experiments showed thatthis material was not inert and caused some coking. The later studieswere conducted using 1/8-inch long quartz billets cut from 2 mm rod as asupport.

Special precautions were taken to exclude oxygen from the apparatus andto keep the stilbene feed in the vapor phase. Special steam tracing,heating, and nitrogen purging of lines contacting stilbene are essentialin a pilot plant, but may not be as critical in a large scale commercialplant.

Catalyst Preparation

A series of catalysts was prepared. The basic catalyst contained 0.56 wt% WO₃ and 0.038 wt % K₂ O. The catalyst was prepared by adding 20 g of14-35 mesh Davison Grade 59 silica gel, which had been freshly calcined,to 28 ml of a solution containing 5 ml of 0.0236N KOAc solution, 10 mlof H₂ O, and 13 ml of concentrated NH₄ OH. The silica gel was onlyminimally wetted by the 28 ml of liquid. The mixture was shaken for 30minutes, then dried overnight in a stream of air on a filter, andfinally calcined for 2 hours at 600 C. Various additives, those whichwere soluble in the alkaline solution described above, were simply addedto the alkaline solution along with the potassium and tungstencomponents. In some cases, because of solubility limitations, ammoniumhydroxide would not dissolve the additive, so in these cases a few dropsof concentrated HNO₃ was added to obtain a clear solution. In all casesthe total liquid volume of impregnating solution was 28 ml, the exactvolume was obtained by adjusting the amount of water added. In allcases, except where noted, additives were added sufficient to give anatomic ratio of tungsten:additive of 5:1. I believe the additives, theadded metallic components, were present as oxides on the catalysts,because of the calcination in air for two hours at 600 C.

When iridium or rhodium were added, a different procedure was used as nowater soluble iridium or rhodium compounds were readily available. Alarge batch of base catalyst (containing 0.56 wt % WO₃ and 0.038 wt % K₂O) was made up as described above. A 20.12 g portion of this catalystwas then impregnated with 25 ml of a methanol solution containing 0.0336g of Ir(CO)₂ acac or 0.0375 g of Rh (acac). This alcoholic impregnatingsolution was sufficient to just impart wetness to the catalyst. Aftershaking for 30 minutes, drying in air, and calcining for 2 hours at 600C. the catalysts were ready for use.

Table I shows a listing of catalysts prepared.

                                      TABLE I                                     __________________________________________________________________________    PREPARATION OF 0.56% WO.sub.3, 0.038% K.sub.2 O CATALYSTS WITH                ADDED METALLIC COMPONENTS                                                          Compound   Wt of                                                         Additives                                                                          Used       Compound.sup.a, g.                                                                    Comments                                              __________________________________________________________________________    Pt   Pt(NH.sub.3).sub.2 (ONO).sub.2                                                           0.0330.sup.b                                                  Pd   Pd(NH.sub.3).sub.2 (ONO).sub.2                                                           0.0232  5 drops HNO.sub.3, boiled to dissolve salts           Ni   Ni(NO.sub.3).sub.3.6H.sub.2 O                                                            0.0281  No NH.sub.4 OH                                        Zn   Zn(OAc).sub.2.2H.sub.2 O                                                                 0.0212  No NH.sub.4 OH; 5 drops 30% H.sub.2 O.sub.2           Cr   Cr(NO.sub.3).sub.3.9H.sub.2 O                                                            0.0387  "                                                     Fe   Fe(NO.sub.3).sub.3.9H.sub.2 O                                                            0.0390  No NH.sub.4 OH; 5 drops 30% H.sub.2 O.sub.2 ; 10                              drops HNO.sub.3                                       Ru   RuNO(NO.sub.3).sub.3                                                                     0.0306.sup.c                                                                          5 drops 30% H.sub.2 O.sub.2                           Mo   (NH.sub.4).sub.6 Mo.sub.7 O.sub.2 4.4H.sub.2 O                                           0.0171                                                        V    NH.sub.4 VO.sub.3                                                                        0.0114.sup.d                                                  Sn   SnSO.sub.4 0.0218  No NH.sub.4 OH; 10 drops conc. H.sub.2 SO.sub.4,                              4 drops HNO.sub.3                                     Re   Re.sub.2 O.sub.7.3 (Dioxane)                                                             0.0362  No NH.sub.4 OH; 5 drops 30% H.sub.2 O.sub.2           Ag   AgNO.sub.3 0.0164                                                        Ce   Ce(NO.sub.3).sub.3.6H.sub.2 O                                                            0.0419  No NH.sub.4 OH                                        Eu   Eu(NO.sub.3).sub.3.6H.sub.2 O                                                            0.0431  No NH.sub.4 OH                                        As   As.sub.2 O.sub.5.nH.sub.2 O                                                              0.0127.sup.e                                                                          No NH.sub.4 OH                                        U    UO.sub.2 (C.sub.2 H.sub.3 O.sub.2).sub.2.2H.sub.2 O                                      0.0410  No NH.sub.4 OH                                        Mn   Mn(C.sub.2 H.sub.3 O.sub.2).sub.2.4H.sub.2 O                                             0.0237  No NH.sub.4 OH                                        Rh   Rh(acac)   0.0375  Alcoholic impregnation                                Ir   Ir(CO).sub.2 acac                                                                        0.0336  Alcoholic impregnation                                __________________________________________________________________________     .sup.a 20 g of silica gel base                                                .sup.b 61.00% Pt                                                              .sup.c 35.87% Ru                                                              .sup.d 76.90% V.sub.2 O.sub.5                                                 .sup.e 8765% As.sub.2 O.sub.5                                            

Catalyst Activation

Catalysts were activated, in situ, by passing 200 scc/min of CO over thecatalyst at a specified temperature for specified time. It is possibleto use other activating gases, or no gas at all, but a CO activationprocedure was chosen as a standard one to permit screening of theeffects of various additives on catalyst activation.

Test Procedure

The activated catalyst was then tested for its activity on a standardfeed consisting of 200 scc/min ethylene and 40 scc/min of stilbene. Theresidence time in the catalyst bed was 0.3 seconds. The products wereanalyzed by gas chromatography.

After the catalyst lost activity, it was regenerated by contacting itwith 48 scc/min of air for 45 minutes at 575 C.

A typical operating sequence is presented below:

A. Activation Cycle

1. 25 min. Nitrogen purge of lines and reactor system (200 cc/min).Ethylene purge of line up to oxygen trap. Reactor temperatureequilibrated to activation temperature.

2. 5 min. Nitrogen purge continuing. Ethylene purge of lines throughoxygen trap to vent, located at ethylene-to-saturator feed valve. COflow to vent to purge CO line in panel control board.

3. 55 min., typical. Nitrogen off. CO feed to reactor for activation,feed rate typically 200 cc/min. Ethylene purge to vent continuing, withoxygen meter (Teledyne Trace Oxygen Analyzer Model 311-1) connected tovent to monitor ethylene quality.

4. 5 min. Nitrogen purge to vent to clear lines in control panel. COfeed to reactor and ethylene feed to vent continuing.

5. 15 min. Nitrogen purge of reactor and lines. Ethylene feed to ventcontinuing. Temperature changed to disproportionation run temperature.

6. 5 min. Ethylene feed to reactor, bypassing saturator. Saturator feedvalve open to reactor to equalize pressure.

B. Disproportionation Cycle

1. 30 min. Ethylene feed through stilbene saturator and thence toreactor; GC sampling program called during last 60 sec. of cycle. Stepis repeated as desired.

C. Burn-off Cycle

1. 5 min. Nitrogen purged to vent to clear lines in control panel.Ethylene feed to saturator off, but saturator feed valve to reactor opento equalize pressure.

2. 60 min. Nitrogen purge to reactor (48 cc/min). Temperature changed toburn-off temperature, usually 575 C. Air purged to vent to equilibratepressure in line.

3. 45 min. Air feed to reactor, 48 cc/min. GC analysis for CO₂ called.

4. 15 min. Nitrogen purge, 200 cc/min., through reactor system.

5. Shut down or recycle.

This procedure was used to test the different catalyst formulations.Experimental results are shown as productivity, measured as moles ofstyrene per liter of catalyst per hour. 34 moles per liter per hourrepresents about 83% conversion of stilbene to styrene. Productivity isreported both for the start of run conditions (initial) and at the endof the run, i.e., after 4.5 hours of operation (final). The data arepresented below in Table IIA.

                  TABLE IIA                                                       ______________________________________                                        EFFECTIVE OF ADDITIVES                                                        ON CATALYST ACTIVATION                                                                    Activation                                                        Catalyst.sup.a                                                                            Conditions   Productivity.sup.b                                   Additive    Time   Temp.     Initial                                                                              Final                                     ______________________________________                                        None (STD)  None         4.19     6.74                                                    1 hr   450       8.05   12.65                                                 8 hr   450       27.65  19.43                                     Ce          1 hr   450       8.13   12.30                                     Eu          None         4.01     6.27                                                    1 hr   450       6.91   12.57                                     As          None         2.16     3.99                                                    1 hr   450       8.62   12.91                                                 1 hr   450       8.92   13.31                                     Fe          1 hr   450       9.07   17.79                                     Cr          1 hr   450       7.10   12.20                                     Ni          1 hr   450       7.51   13.82                                     Ru          1 hr   450       4.68   11.19                                     Ir          1 hr   450       17.02  15.18                                                 8 hr   450       31.35  22.77                                     Pt          1 hr   450       9.38.sup.c                                                                           7.21.sup.c                                Pd          1 hr   450       7.29.sup.c                                                                           6.35.sup.c                                ______________________________________                                         .sup.a All additives at 5:1 W: additive mole ratio unless otherwise noted     .sup.b All runs were for 4.5 hr. Run temp., 425 C.                            .sup.c Average for 2 runs.                                               

The test apparatus was then partially dismantled and rebuilt. A numberof additional tests were then run. The main difference betweenoperations reported in Table IIA and Table IIB, presented hereafter, isthe amount of oxygen contamination. I believe that the data presented inIIA reflect less oxygen contamination than those in Table IIB. Since thetesting occurred under super atmospheric pressure, it was thought thatthere could be no air contamination due to leaks in the piping.Reactants might leak out, but air would not get in. Several ppm oxygendiffused into the test apparatus through a leak to increase the oxygenlevel, and decrease the catalyst activity. Oxygen is a catalyst poison.The amount of O₂ contamination was relatively constant during the IIAtesting period, I estimate about 0.2 ppm O₂ by volume. For the IIBtesting period about 0.3 ppm O₂ by volume was present. I checked theactivity of my standard, or reference, catalyst periodically during theIIA and IIB testing periods. The standard, or reference, catalystconsistently gave lower productivity during the IIB tests. The resultsof the more O₂ contaminated runs are reported in Table IIB. The pressurefor tests reported in Tables IIA and IIB was 3 psig, or 1.2 atm,absolute.

                  TABLE IIB                                                       ______________________________________                                        EFFECT OF ADDITIVES                                                           ON CATALYST ACTIVATION                                                                    Activation                                                        Catalyst.sup.a                                                                            Conditions    Productivity                                        Additive    Time   Temp.      Initial                                                                             Final                                     ______________________________________                                        None (STD)  1 hr   450        6.09  11.04                                                 1 hr   450        5.87  11.49                                                 1 hr   450        4.53  9.63                                      Rh          1 hr   450        9.82  10.60                                                 1 hr   450        2.98  8.16                                                  1 hr   450        2.41  7.45                                                  8 hr   450        2.76  6.53                                      Mo          1 hr   450        3.92  7.75                                                  1 hr   450        4.75  7.56                                      V           1 hr   450        1.05  2.86                                      Sn          1 hr   450        0.04  (0.18)                                                1 hr   450        1.34  (1.61)                                    Re          1 hr   450        4.49  4.26                                      Zn          1 hr   450        2.82  4.32                                      Ag          1 hr   450        3.42  8.02                                                  1 hr   450        4.32  8.48                                      U           1 hr   450        4.98  5.47                                      Mn          1 hr   450        3.76  4.04                                      ______________________________________                                         .sup.a All additives at 5:1 W: additive mole ratio otherwise noted.      

It is believed that results can be compared very well within Table IIA,or within Table IIB. Direct comparison of an additive listed in TableIIB with an additive from Table IIA is harder to make, because of theincreased oxygen contamination in those runs presented in Table IIB. Itis believed that the relative activities, i.e., activity of a catalystin Table IIB with an additive compared to activity of a catalyst with noadditives from Table IIB can be compared. These data, relativeactivities, for initial activity, are reported in Table III. Therelative initial activities are probably more significant than relativeend of run activities, so comparisons were made based on relativeinitial activities.

                  TABLE III                                                       ______________________________________                                        Additive           Relative Activities                                        ______________________________________                                        Ir                 2.11, 2.03                                                 Fe                 1.13                                                       As                 1.10                                                       Pt                 1.09                                                       Ce                 1.01                                                       Rh                 1.10, 0.56, 0.45                                           U                  1.03                                                       None (Standard)    1.00                                                       Re                 0.93                                                       Ag                 0.89, 0.71 -Mo 0.89, 0.74                                  Ni                 0.93, 0.77                                                 Cr                 0.88                                                       Eu                 0.86                                                       Pd                 0.85                                                       Mn                 0.78                                                       Bi(0.56%) (1:1)    0.71, 0.50                                                 Zn                 0.58                                                       Ru                 0.58                                                       Sn                 0.31                                                       V                  0.16, 0.17, 0.20, 0.20                                     ______________________________________                                    

From these data, it is apparent that a reverse disproportionationcatalyst containing tungsten, potassium and iridium gives excellentresults with only a slight activity loss after regeneration andactivation. The W:Rh catalyst lost activity with each regenerationcycle.

It is possible to obtain even higher productivity from the iridiumcatalyst than reported above. The above data are based on activation at450 C., a relatively low activation temperature chosen to mesh with thereverse disproportionation reaction temperature. By going to a moresevere activation, for one hour at 575 C. with CO, even higherproductivities can be obtained, with both the standard catalyst(tungsten and potassium on silica gel) and iridium, tungsten andpotassium catalyst.

With the iridium, tungsten and potassium catalyst, the initialproductivity, after a 575 C. activation, was 34.10 moles of styrenemonomer per liter of catalyst per hour, roughly 83% conversion. Reactionconditions at start and end of run were a feed rate of 200 scc/min ofethylene, 40 scc/min of stilbene at 425 C. The estimated residence timewithin the reactor was 0.3 seconds. After operation for 12 hours, thecatalyst productivity was 29.61. The % coke on catalyst was 0.11 wt %.

In a similar testing procedure, the standard catalyst, after a 575 C.activation, had an initial productivity of 33.73 moles of styrenemonomer per liter of catalyst per hour and a final productivity of26.48. The coke on standard catalyst after 12 hours of operation was0.43 wt %.

With the more severe activation, the iridium-tungsten catalystapproached equilibrium conversion, 83% of styrene, as did the standardcatalyst. Both were close to equilibrium conversion, so initialactivities don't show much difference between these catalysts. After 12hours of operation the iridium-tungsten catalyst had a significantlyhigher productivity than did the standard catalyst. There was less cokeon the iridium-tungsten catalyst. Directionally, this indicates that theiridium-tungsten catalyst might last much longer in commercial operationthan a tungsten catalyst. I do not put too much faith, however, in thenumbers presented for coke levels. The coke levels are generally low andthe differences between coke levels in these catalysts may approach theexperimental error of my coke measurement system. Some tests on othercatalysts containing other promoters showed that coke level, asdetermined by my experimental methods, did not correlate very well withinitial and final productivities. Phrased another way, other catalystshad relatively low coke make, but none of them had productivities ashigh as the iridium-promoted tungsten potassium on silica catalysts.

I am not sure what the optimum amount of iridium is. I know a 1:5 Ir:Wratio gives good results.

I believe that good results can be obtained with Ir:W atom ratios of1:20 to 2:1 and preferably 1:10 to 1:3.

If I were designing a commercial plant today, I would conduct furtherexperiments to see if the various catalytic components could beoptimized further. I would probably use a catalyst containing 2 to 10times as much metal content as those catalysts used in the experiments.Commercially, you want more active catalysts, and smaller reactionvessels, and would use catalysts with a higher metal loading. I usedvery lightly loaded catalysts for my experiments because the catalystwas extremely active. "Full strength" catalyst established equilibriumconditions so rapidly that I could not discern relatively smallerdifferences caused by different additives. Based on other experimentalwork, metal loadings ten times as high can probably be achieved usingsimilar impregnation procedures, with five or tenfold increase inactivity. Phrased another way, the reactants see the active metals, notthe support, and the amount of conversion per gram of catalyticcomponents (excluding support) is roughly constant. More metal on thesupport should improve the catalyst's resistance to trace amounts of O₂and polars.

I would like to learn more about the active form of the catalysts Itested. The active form may be a simple oxide or may be a mixedheteropolyacid of SiO₂, WO₃ and MO_(x), where M is the additive metal.It is possible that the oxides mentioned and claimed do not exist asdiscrete oxides, but instead form some complex polymeric structure.

I would operate a commercial plant with whatever oxygen strippingcolumns or oxygen and water absorbers were necessary to ensure oxygen,and other contaminants, especially polar ones, were excluded from theplant.

My catalyst can be disposed within the reactor as a fixed bed, fluidizedbed, moving bed, ebullated bed, or any other reactor configuration. Theadvantage of the fluidized, moving and ebullating bed reactors is thatcatalyst addition and withdrawal can be performed continuously. Thus,coke, or carbon deposition on the catalyst can be burned off, thecatalyst activated, and returned to the reactor without shutting downthe reactor. The disadvantage of this mode of operation is that thereactor designs are fairly complicated, as compared to simple fixed-bed,down-flow design. When fixed-bed reactors are used, preferably, two orthree reactors are provided in parallel, permitting one or more reactorsto be taken off stream for carbon burn-off and activation while theother reactor(s) remain on stream.

I claim:
 1. A catalyst comprising catalytically-effective amounts ofiridium, tungsten, and an alkali or alkaline earth component on acarrier material.
 2. Catalyst of claim 1 wherein the carrier material issilica gel.
 3. Catalyst of claim 1 wherein the alkali or alkaline earthcomponent is potassium.
 4. Catalyst of claim 1 wherein the catalystcontains 0.1 to 10 wt % W, 0.01 to 2 wt % Ir, and 0.01 to 2 wt % alkalior alkaline earth component.
 5. Catalyst of claim 1 wherein the catalystcontains 1 to 6 wt % W as WO₃, 0.1 to 1 wt % Ir as IrO₂ or Ir₂ O₃, and0.03 to 0.3 wt % K₂ O, and the catalyst support is silica gel. 6.Catalyst of claim 1 wherein the Ir:W atomic ratio is 1:20 to 2:1. 7.Catalyst of claim 1 wherein the Ir:W atomic ratio is 1:5.